Valve trim for a flow control valve typically refers to the internal components of a valve that modulate the flow of the controlled fluid. The components of the valve trim vary from valve type to valve type. However, a valve trim typically includes at least a flow control orifice and a flow control member that cooperates with the flow control orifice to control the flow of fluid through the orifice and, subsequently, through the valve. Many valve trims include additional components, such as a valve stem, a cage, and/or other internal components. However, the focus of this application is primarily on the flow control member and the flow control orifice, it being understood that a valve trim may include additional components.
A high recovery valve is a valve design that dissipates relatively little flow stream energy due to streamlined internal contours and minimal flow turbulence. A high recovery valve typically includes a flow control member in the form of a valve plug and an orifice with a generally hourglass-shaped inner wall surface. Thus, the geometry of a high recovery valve typically simulates a converging-diverging nozzle. For example, FIG. 1 illustrates a generic converging-diverging nozzle geometry and shows the fluid flow through the restriction and the vena contracta with an inlet pressure upstream of the restriction, i.e., stagnation pressure P1, and an outlet pressure downstream of the restriction, i.e., back pressure P2.
In a high recovery valve, pressure downstream of the vena contracta recovers to a high percentage of its inlet value. FIG. 2 illustrates a comparison of typical pressure profiles for high and low recovery valves. As can be seen in FIG. 2, in both high and low recovery valves there is a pressure differential dP between the stagnation pressure P1 and the back pressure P2 across the valve throat. Thus, dP=P1−P2. Here, the valve throat is modeled as a simple vena contracta. However, the back pressure P2 of the high recovery valve returns closer to the stagnation pressure P1 than the back pressure of the low recovery valve. Nevertheless, the pressure differential dP across the valve throat creates a stem force Fs on a valve stem attached to a valve plug at the valve throat.